[llvm-reduce] Fix incorrectly ignored null MD in ReduceDIMetadata (llvm#108541)

Commit c2e62c7 updated the ReduceDIMetadata pass to be able to remove
DIGlobalVariableExpressions from MDNode operands; it also accidentally
prevented null operands from being preserved, which results in an
assertion being triggered:
`Targets == NoChunksCounter.count() && "number of chunks changes when
reducing"'

This patch allows us to correctly preserve null operands once again.
I've not got a test case for this yet - I'm hoping this patch is just
trivially correct as-is, because I've not got the hang of reducing a
test case for llvm-reduce yet, but I can get a test case generated if
needed.

Author: SLTozer

llvm/test/tools/llvm-reduce/108541-null-metadata-crash.ll

@@ -0,0 +1,47 @@
+; RUN: llvm-reduce %s -o %t --delta-passes=di-metadata --test FileCheck --test-arg %s --test-arg --input-file --abort-on-invalid-reduction
+; CHECK: , !dbg !11
+
+;; Tests for the bug fixed in PR#108541, where the presence of null metadata
+;; could result in a crash.
+
+define i1 @ham() {
+bb:
+  %call = call fastcc i32 @hoge()
+  ret i1 false
+}
+
+define fastcc i32 @hoge() {
+bb:
+  br i1 poison, label %bb1, label %bb2
+
+bb1:                                              ; preds = %bb
+  br i1 false, label %bb2, label %bb2, !dbg !11
+
+bb2:                                              ; preds = %bb1, %bb1, %bb
+  ret i32 0
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!10}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !1, producer: "clang version 20.0.0git", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, retainedTypes: !2, globals: !2, imports: !3, splitDebugInlining: false, nameTableKind: None)
+!1 = !DIFile(filename: "108541-metadata-crash.cpp", directory: "/tmp")
+!2 = !{}
+!3 = !{!4}
+!4 = !DIImportedEntity(tag: DW_TAG_imported_declaration, scope: !5, entity: !6, file: !7, line: 134)
+!5 = !DINamespace(name: "std", scope: null)
+!6 = !DISubprogram(name: "abort", scope: !7, file: !7, line: 730, type: !8, flags: DIFlagPrototyped | DIFlagNoReturn, spFlags: DISPFlagOptimized)
+!7 = !DIFile(filename: "108541-metadata-crash.cpp", directory: "")
+!8 = !DISubroutineType(types: !9)
+!9 = !{null}
+!10 = !{i32 2, !"Debug Info Version", i32 3}
+!11 = !DILocation(line: 26, column: 11, scope: !12)
+!12 = distinct !DILexicalBlock(scope: !14, file: !13, line: 26, column: 11)
+!13 = !DIFile(filename: "108541-metadata-crash.cpp", directory: "/tmp")
+!14 = distinct !DILexicalBlock(scope: !15, file: !13, line: 25, column: 5)
+!15 = distinct !DILexicalBlock(scope: !16, file: !13, line: 24, column: 9)
+!16 = distinct !DILexicalBlock(scope: !17, file: !13, line: 14, column: 3)
+!17 = distinct !DILexicalBlock(scope: !18, file: !13, line: 13, column: 3)
+!18 = distinct !DILexicalBlock(scope: !19, file: !13, line: 13, column: 3)
+!19 = distinct !DISubprogram(name: "hoge", linkageName: "hoge", scope: !13, file: !13, line: 10, type: !20, scopeLine: 11, flags: DIFlagPrototyped | DIFlagAllCallsDescribed, spFlags: DISPFlagLocalToUnit | DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !2)
+!20 = distinct !DISubroutineType(types: !2)

llvm/tools/llvm-reduce/deltas/ReduceDIMetadata.cpp

@@ -67,8 +67,8 @@ void identifyUninterestingMDNodes(Oracle &O, MDNodeList &MDs) {
           // Don't add uninteresting operands to the tuple.
           if (!O.shouldKeep())
             continue;
-        TN.push_back(Op);
       }
+      TN.push_back(Tup->getOperand(I));
     }
     if (TN.size() != Tup->getNumOperands())
       DbgNode->replaceOperandWith(OpIdx, DbgNode->get(DbgNode->getContext(), TN));